And this is how it always is. The archaeologists never find anything weird or unique. All the things that come out of the earth- floor-plans, mosaics, decorative schemes, doodads- always conform to recognisable type.
The way they're put together may vary- but the elements remain the same. Roman art and design didn't come off a production line- it wasn't mass-produced- but it might as well have been for all the variety on show.
In theory the Roman landowner could have sat down with his architect and said, "Look, I want to think outside the box here- how about something in the Egyptian taste?"- but he never did.
It wasn't that other styles weren't available, it was just that it never occured to anyone to risk the charge of eccentricity.
I was going to opine that the Romans were a peculiarly conservative people- but, then I thought about it and realised they weren't. Our own age is no different. We have a much wider range of architectural styles to draw on than the Romans did but we mainly don't employ them. We could be building gothic churches and classical law courts but we don't because that would be old-fashioned. And we could be building ultra modern, eco-friendly town houses but we don't because they're weird-looking.
And we don't care to be laughed at.
Every culture is pretty much like this when it comes to architecture- and the arts in general- but architecture in particular because buildings are so public and represent such a huge investment of time and money.